INTRODUCTION 



Concrete is the most common construction material used by the Navy 

 in waterfront structures. It is estimated that more than 40% of Navy 

 piers consist of a concrete deck supported by concrete piles (Ref 1) . 

 In addition, concrete is also used extensively for retaining walls, 

 encasement of other materials such as steel piles, and pavement. To 

 adequately maintain these structures, periodic inspections are required, 

 both above and below water. 



Currently, underwater inspections of concrete structures are con- 

 ducted visually to assess the condition of the facility. The qualitative 

 data obtained from these inspections are generally inadequate to accur- 

 ately assess the condition of the structure. New techniques and equip- 

 ment are required to provide more quantitative data from underwater 

 inspections of concrete structures. 



The Naval Civil Engineering Laboratory (NCEL) , under the sponsorship 

 of the Naval Facilities Engineering Command (NAVFAC) , has initiated a 

 project to assess potential techniques for nondestructive testing (NDT) 

 of concrete underwater. This report presents the results of laboratory 

 and field evaluations of the Schmidt hammer, magnetic rebar locator, and 

 ultrasonic testing equipment, all of which were modified for underwater 

 use. 



BACKGROUND 



Many techniques for testing concrete above water have been developed 

 and generally are well documented in the literature. Most of these tech- 

 niques are discussed in Reference 2, published by the American Concrete 

 Institute, which provides a good summary of nondestructive methods of 

 testing concrete on land. Those techniques most easily adapted for 

 inspecting concrete structures underwater have been identified and are 

 listed below (Ref 3) . 



• Magnetic Rebar Location - Magnetic rebar location devices detect 

 the distortion in a magnetic flux field caused by the pressence of 

 metallic rebar. 



• Rebound Method - The compressive strength of the concrete is corre- 

 lated with the rebound height of a spring driven mass after impact. 



Ultrasonic Testing - The transit time of high frequency sound waves 

 is used to assess the condition of the concrete and detect internal 

 defects. 



